x^2-4x=60
If each row has the same number of chairs as rows, then there is x^2 chairs.
Because there is 4 fewer chairs in every row, for every row you minus 4 - 4x
There a 60 chairs in total.
x^2 - 4x = 60
Substitute a number into the formula to check:
If there are 10 rows:
x = 10
10^2 - 4(10) = 60
100 - 40 = 60.